Seismic hazard impact of the Lower Tagus Valley Fault Zone (SW Iberia)

Abstract The Vila Franca de Xira (VFX) fault is a regional fault zone located about 25 km northeast of Lisbon, affecting Neogene sediments. Recent shear-wave seismic studies show that this complex fault zone is buried beneath Holocene sediments and is deforming the alluvial cover, in agreement with a previous work that proposes the fault as the source of the 1531 Lower Tagus Valley earthquake. In this work, we corroborate these results using S-wave, P-wave, geoelectric, ground-penetrating radar and borehole data, confirming that the sediments deformed by several fault branches are of Upper Pleistocene to Holocene. Accumulated fault vertical offsets of about 3 m are estimated from the integrated interpretation of geophysical and borehole data, including 2D elastic seismic modeling, with an estimated resolution of about 0.5 m. The deformations affecting the Tagus alluvial sediments probably resulted from surface or near-surface rupture of the VFX fault during M∼7 earthquakes, reinforcing the fault as the seismogenic source of regional historical events, as in 1531, and highlighting the need for preparedness for the next event.

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Near‐Surface Characterization of the Lisbon and Lower Tagus Valley Area, Portugal, for Seismic Hazard Assessment: VS30 and Soil Classification Maps

Bulletin of the Seismological Society of America ◽

10.1785/0120170340 ◽

2018 ◽

Vol 108 (5A) ◽

pp. 2854-2876

Author(s):

J. Carvalho ◽

R. Dias ◽

R. Ghose ◽

P. Teves‐Costa ◽

J. Borges ◽

...

Keyword(s):

Seismic Hazard ◽

Hazard Assessment ◽

Surface Characterization ◽

Seismic Hazard Assessment ◽

Soil Classification ◽

Near Surface ◽

Lower Tagus Valley ◽

Tagus Valley ◽

Valley Area

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Large Holocene Earthquakes in the Lower Tagus Valley Fault Zone, Central Portugal

Seismological Research Letters ◽

10.1785/gssrl.83.1.67 ◽

2012 ◽

Vol 83 (1) ◽

pp. 67-76 ◽

Cited By ~ 14

Author(s):

G. M. Besana-Ostman ◽

S. P. Vilanova ◽

E. S. Nemser ◽

A. Falcao-Flor ◽

S. Heleno ◽

...

Keyword(s):

Fault Zone ◽

Lower Tagus Valley ◽

Central Portugal ◽

Tagus Valley

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The Azambuja fault: An active structure located in an intraplate basin with significant seismicity (Lower Tagus Valley, Portugal)

Journal of Seismology ◽

10.1023/b:jose.0000038450.23032.68 ◽

2004 ◽

Vol 8 (3) ◽

pp. 347-362 ◽

Cited By ~ 31

Author(s):

J. Cabral ◽

P. Ribeiro ◽

P. Figueiredo ◽

N. Pimentel ◽

A. Martins

Keyword(s):

Active Structure ◽

Lower Tagus Valley ◽

Tagus Valley

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Fractal properties and simulation of micro-seismicity for seismic hazard analysis: a comparison of North Anatolian and San Andreas Fault Zones

Research in Geophysics ◽

10.4081/rg.2012.e1 ◽

2012 ◽

Vol 2 (1) ◽

pp. 1 ◽

Cited By ~ 3

Author(s):

Naside Ozer ◽

Savas Ceylan

Keyword(s):

Fractal Dimension ◽

Seismic Hazard ◽

Fault Zone ◽

San Andreas Fault ◽

Fractal Dimensions ◽

Fault Zones ◽

Group Method ◽

Western Anatolia ◽

P Value ◽

San Andreas

We analyzed statistical properties of earthquakes in western Anatolia as well as the North Anatolian Fault Zone (NAFZ) in terms of spatio-temporal variations of fractal dimensions, p- and b-values. During statistically homogeneous periods characterized by closer fractal dimension values, we propose that occurrence of relatively larger shocks (M >= 5.0) is unlikely. Decreases in seismic activity in such intervals result in spatial b-value distributions that are primarily stable. Fractal dimensions decrease with time in proportion to increasing seismicity. Conversely, no spatiotemporal patterns were observed for p-value changes. In order to evaluate failure probabilities and simulate earthquake occurrence in the western NAFZ, we applied a modified version of the renormalization group method. Assuming an increase in small earthquakes is indicative of larger shocks, we apply the mentioned model to micro-seismic (M<= 3.0) activity, and test our results using San Andreas Fault Zone (SAFZ) data. We propose that fractal dimension is a direct indicator of material heterogeneity and strength. Results from a model suggest simulated and observed earthquake occurrences are coherent, and may be used for seismic hazard estimation on creeping strike-slip fault zones.

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Aeromagnetic Interpretations of the Crittenden County Fault Zone

Seismological Research Letters ◽

10.1785/0220200209 ◽

2020 ◽

Vol 92 (1) ◽

pp. 494-507

Author(s):

Christopher Marlow ◽

Christine Powell ◽

Randel Cox

Keyword(s):

Seismic Hazard ◽

Fault Zone ◽

Detection Methods ◽

Magnetic Data ◽

Horizontal Gradient ◽

Reverse Fault ◽

Maximum Magnitude ◽

Fault Systems ◽

Basement Faults ◽

Seismic Reflection Profiles

Abstract The Crittenden County fault zone (CCFZ) is a potentially active fault zone located within 25 km of Memphis, Tennessee, and poses a significant seismic hazard to the region. Previous research has associated the fault zone with basement faults of the eastern Reelfoot rift margin (ERRM) and described it as a northeast-striking, northwest-dipping reverse fault. However, we suggest that there is an incomplete understanding of the fault geometry of the CCFZ and the ERRM in this region due to significant gaps in seismic reflection profiles used to interpret the fault systems. To improve our understanding of the structure of both fault systems in this region, we apply two processing techniques to gridded aeromagnetic data. We use the horizontal gradient method on reduction-to-pole magnetic data to detect magnetic contacts associated with faults as this technique produces shaper gradients at magnetic contacts than other edge detection methods. For depth to basement estimations, we use the analytic signal as the method does not require knowledge of the remnant magnetization of the source body. We suggest that the CCFZ extends approximately 16 km farther to the southwest than previously mapped and may be composed of three independent faults as opposed to a continuous structure. To the northeast, we interpreted two possible faults associated with the ERRM that intersect the CCFZ, one of which has been previously mapped as the Meeman–Shelby fault. If the CCFZ and the eastern rift margin are composed of isolated fault segments, the maximum magnitude earthquake that each fault segment may generate is reduced, thereby, lowering the existing seismic hazard both fault systems pose to Memphis, Tennessee.

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